Alkylene glycols, in particular monoalkylene glycols, are of established commercial interest. For example, monoalkylene glycols are being used in anti-freeze compositions, as solvents and as base materials in the production of polyalkylene terephthalates e.g. for fibers and bottles.
The production of alkylene glycols by liquid phase hydrolysis of alkylene oxides is known. In commercial production the hydrolysis is performed without a catalyst by adding a large excess of water, e.g. 15 to 30 moles of water per mole of alkylene oxide. The reaction is considered to be a nucleophilic substitution reaction, whereby opening of the alkylene oxide ring occurs, water acting as the nucleophile. Because the primarily formed monoalkylene glycol also acts as a nucleophile, as a rule a mixture of monoalkylene glycol, dialkylene glycol and higher alkylene glycols is formed. In order to increase the selectivity to monoalkylene glycol, it is necessary to suppress the secondary reaction between the primary product and the alkylene oxide, which competes with the hydrolysis of the alkylene oxide.
One effective means for suppressing the secondary reaction is to increase the relative amount of water present in the reaction mixture. Although this measure improves the selectivity towards the production of the monoalkylene glycol, it creates a problem in that large amounts of water have to be removed for recovering the product. Removing this additional water increases production costs as it is energy intensive and requires large-scale distillation facilities.
The demand for monoalkylene glycols has risen significantly in recent years and further growth is expected on account of the increasing popularity of monoalkylene glycol derived products. Most existing commercial alkylene glycol production facilities already operate at or close to maximum (design) capacity. Therefore, to meet the increased demand more efficient methods of producing monoalkylene glycols are required.
In commercial thermal alkylene glycols production processes, the limiting factor on the amount of monoalkylene glycol production is frequently the distillation of water from the aqueous glycol reactor product, as removing the large amounts of water required for high selectivity is a relatively lengthy process. This is problematic as the distillation step acts as a bottleneck, restricting the overall amount of production.
One method of overcoming this problem would be to reduce the ratio of water to alkylene oxide employed in the process. However, this would also increase the relative yield of less desirable higher alkylene glycol products, and possibly necessitate an expansion of facilities to remove and purify the higher alkylene glycol products from the monoalkylene glycol product.
Due to the size and cost of distillation and purification apparatus required to remove water and/or higher glycols, increasing distillation capacity is in many cases neither a practical nor cost-effective solution. Accordingly, it would be advantageous if there was a flexible means with which to overcome this problem such that glycol production could be increased while retaining high selectivity to monoalkylene glycol products.
Catalytic processes for converting alkylene oxides to alkylene glycols have been investigated and catalysts capable of promoting a higher selectivity to monoalkylene glycol product at reduced water levels are known, (e.g. EP-A 015649, EP-A 0160330, WO 95/20559 and U.S. Pat. No. 6,124,508). For some catalysts, such as the quaternary phosphonium cation-containing catalysts of U.S. Pat. No. 6,124,508, it is mentioned that in order to save the catalyst it may be advantageous to subject the alkylene oxide feed stream to partial thermal hydrolysis before completing the hydrolysis catalytically.